V I S H AY I N T E R T E C H N O L O G Y, I N C. Circuit Protection with Non-Linear Resistors PTC and NTC Thermistors, Through-Hole Varistors, SMD Thin-Film Chip Fuses r e s i s t i v e p r o d u c t s c a p a b i l i t i e s w w w. v i s h a y. c o m
Circuit Protection with Non-Linear Resistors Using PTC Positive Temperature Coefficient Thermistors The electrical resistance of PTC (positive temperature coefficient) thermistors increases and decreases as the ambient temperature increases and decreases. PTC thermistors are used for voltage and/or current overload protection, temperature protection, inrush current generation, and as a heating element. They protect a wide range of circuits, including line cards, set-top boxes, and private automated branch exchanges in telecom applications; airbag and temperature control devices in automobiles; power supplies and battery chargers in electronic data processing applications; and in other consumer products. PTC Protection Principles Features Different voltages in function of the application: 20 V, 30-60 V, 145 V, 265 V Several executions: naked, leaded, taped, SMD leadframe Wide range of trip and hold currents: from 17 ma up to 4.5 A trip currents Wide range of resistance: from 0.16 Ω up to 3 kω Small ratio between trip and hold currents (It/Ih = 1.5 at 25 C) High maximum inrush current: up to 33 A UL approved series Part numbers Overvoltage/overcurrent: 20 V series: 2381 66x 5xxx4 30-60 V series: 2381 66x 5xxx1 (UL approved) 145 V series: 2381 66x 5xxx2 (UL approved) 265 V series: 2381 66x 5xxx3 (UL approved) Overtemperature: Tn: 70 ºC to 170 ºC: 2381 671 91xxx Log R PTC Vsupply Load Normal Operating Zone Figure 1 2Rmin Rmin Protecting Zone 100 time (s) Ts T 10 Log I Itrip Ihold 1 0.1 0 2000 4000 6000 8000 10000 I (ma) When connected in series with the input of an electrical or electronic circuit (see Figure 1), such as a small motor or power supply, the PTC thermistor acts as a self-resettable fuse, protecting the circuit against current, voltage and temperature overload conditions. In normal operating conditions the PTC resistance is low (see Figure 2), and the current is below its hold value (Ihold). However, an overload will quickly heat up the PTC thermistor until, at around switchtemperature (Ts), its resistance becomes high, limiting the current to far below its trip value (Itrip), and so protecting the circuit. Figure 2 V max Log V
Using NTC Negative Temperature Coefficient Thermistors The electrical resistance of NTC (negative temperature coefficient) thermistors increases as the ambient temperature decreases, and decreases when temperature increases. NTC thermistors are used for overtemperature protection in PCs, power supplies, and motherboards; Li-ion battery protection in fast chargers; and in digital scan cameras, fire and smoke detectors, TCXOs, and other automotive, consumer, and industrial applications. They are generally included in dividing or Wheatstone bridges and can provide a measuring voltage to analog-digital converters. They also allow to control the temperature compensation of displays and regulation of temperature with opamps or more complex IC s. Features for Circuit Protection: Several executions: leaded and SMD in cases from 0402 to 1206 Large resistance range: from 3.3 Ω to 470 kω Temperature range: 40 C to 155 C SMD Termination: Ni and pure Sn Customized types available upon request Example of NTC Circuit Protection in PC Cooling Fan Sensor (pos. a) Heatsink Processor Socket Sensor (pos. b) Sensor (pos. c) V C Temperature R=1 S=Q=0 R NTC R high R low A A A C C S R Q Q Fan Motor T High T Low V cc 0 Q Q=1 Fan Off Q=0 On R=S=0 Q=0 (unch.) R=0 S=Q=1 Q=1 Off R=S=0 Q=1 (unch.) Time Time
Using Through-Hole Varistors VDRs (voltage dependent resistors), or varistors, are used for transient surge suppression. Surge suppression circuits are commonly used in computers, automobiles, telecom and industrial equipment, domestic appliances, and other consumer products. Specifications Table Standard Series: 2381 592/593/594/595/596 Sizes from 5 mm to 20 mm V rms from 14 V to 680 V V dc from 18 V to 895 V Can absorb surges up to 6,500 A High-Surge Series: 2381 582/583/584/585/586 Sizes from 5 mm to 20 mm V rms from 11 V to 680 V V dc from 14 V to 895 V Can absorb surges up to 10,000 A Example of VDR Circuit Protection in PC Dangerous Voltage (without VDR) V AB Clamping Voltage Mains Safe Voltage (with VDR) Line Inductance 220 V Mains I Line Capacitance VDR V AB HOME COMPUTER Time Fuse Short Circuit Fuse Opens 1.5 A M Washing Machine Motor 220 W Short Circuit
Circuit Protection with Non-Linear Resistors Using SMD Thin Film Flat Chip Fuse SMD thin film flat chip fuses feature very quick-acting fuse characteristics with outstanding stability, thanks to a highly controlled manufacturing process and Vishay s advanced thin film technology. Available in standard metric case sizes, they are designed to provide secondary overcurrent protection for power inverters, motion-control units, dc-to-dc converters, battery chargers, and low-voltage power supplies in information technology, telecommunications, automotive, medical, and audio/ video systems. Features Available in 0603, 0805, and 1206 cases Standard alumina substrate Low ohmic metal film alloy fuse element Structuring by Photolithography Low ohmic Cu/Ni/pure Sn terminations Epoxy lacquer Fuse current adjustment by precise contour variation of the fuse element geometry (l/b/h) Benefits Fast very quick-acting (FF) High-precision blow characteristics High reliability Stability verified in accordance with IEC 60127-4 and UL 248-14 First chip fuse series in standard SMD sizes approved by UL and VDE Characteristics of SMD Thin Film Chip Fuses t[s] 10 1 0.1 0.01 1E-3 1 E-4 MFU Chip Fuse Series 1 E-5 0 1.25 2 4 6 I R /I pre-arcing time min. pre-arcing time max. 8 10 12 MFU Current Range Table Size Fuse Characteristics Rated Current (A) 1) 0.5 1 1.6 2.0 3.15 4.0 5.0 0402 2) FF X X X X 0603 FF X X X X X X 0805 FF X X X X X X 1206 FF X X X X X X X 1) Other values of rated current are available. Please refer to the MFU Chip Fuse datasheet. 2) MFU 0402 available on request. NOTICE Specifications of the products displayed herein are subject to change without notice. Vishay Intertechnology, Inc., or anyone on its behalf, assumes no responsibility or liability for any errors or inaccuracies. Information contained herein is intended to provide a product description only. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted by this document. Except as provided in Vishay s terms and conditions of sale for such products, Vishay assumes no liability whatsoever, and disclaims any express or implied warranty, relating to sale and/or use of Vishay products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright, or other intellectual property right. The products shown herein are not designed for use in medical, life-saving, or life-sustaining applications. Customers using or selling these products for use in such applications do so at their own risk and agree to fully indemnify Vishay for any damages resulting from such improper use or sale.
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